Semiconductor memory device

ABSTRACT

To increase a storage capacity of a memory module per unit area, and to provide a memory module with low power consumption, a transistor formed using an oxide semiconductor film, a silicon carbide film, a gallium nitride film, or the like, which is highly purified and has a wide band gap of 2.5 eV or higher is used for a DRAM, so that a retention period of potentials in a capacitor can be extended. Further, a memory cell has n capacitors with different capacitances and the n capacitors are each connected to a corresponding one of n data lines, so that a variety of the storage capacitances can be obtained and multilevel data can be stored. The capacitors may be stacked for reducing the area of the memory cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/505,551, filed Oct. 3, 2014, now allowed, which is a continuation ofU.S. application Ser. No. 13/298,235, filed Nov. 16, 2011, now U.S. Pat.No. 8,854,865, which claims the benefit of a foreign priorityapplication filed in Japan as Serial No. 2010-261470 on Nov. 24, 2010,all of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor memory devices.

2. Description of the Related Art

A dynamic random access memory (DRAM) is a semiconductor memory devicewhere one bit of data can be stored with use of one transistor and onecapacitor. The DRAM has a small area per memory cell, easiness inintegration for modularization, and is manufactured at low cost.

In a DRAM, electric charges stored in a capacitor are leaked due tooff-state current of a transistor, and thus an operation of recharge(refresh) is required before charges are lost.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    H6-295589

SUMMARY OF THE INVENTION

A DRAM will be explained with reference to a circuit diagram of FIG. 8A.A DRAM includes a bit line BL, a word line WL, a sense amplifier SAmp, atransistor Tr, and a capacitor C.

It is known that the potential stored in the capacitor is reduced withtime as shown in FIG. 8B. The potential charged from V0 to V1 is reducedwith time, to VA that is a limit for reading out data 1. This period isa retention period T_1. Namely, in the case of a two-level memory cell,a refresh operation is required during the retention period T_1.

On the other hand, there is a limit on the increase of a storagecapacitance of a memory module per unit area only by downsizing a memorycell, and a multilevel technique with which one memory cell contains aplurality of data is desired.

A three-level memory cell from which data 1 and data 2 can be read isdescribed. In FIG. 8C, the limit for reading out data 2 is V1, and theperiod is a retention period T_2. As shown in FIG. 8C, it is found thatthe retention period T_2 is shorter than the retention period T_1 of thetwo-level memory cell. Therefore, the refresh operation is required toperform more frequently. However, it is difficult to realize amultilevel memory cell in consideration of margin of reading.

In the invention disclosed in Patent Document 1, one transistor and aplurality of capacitors are included in one memory cell, and one side ofeach of the plurality of capacitors is electrically connected to asource or a drain of the transistor, and the other sides thereof servesas a plate electrode independently. With the combination of potentialsapplied to the plate electrodes at the time of writing, a plurality ofvalues are memorized in the memory cell.

However, the invention has the following problematic points: (1) theperiod until when a refresh operation is needed is short, and therefresh operation is repeated many times per unit time (which leads toconsumption of a large amount of current), and (2) it is difficult tostore more levels of data.

The point (1) results from the use of a transistor having a channelformed in a semiconductor substrate of silicon in a memory cell. Aslight amount of current flows even in an off-state transistor; in thecase of such a transistor having a channel in a semiconductor substrateof silicon, a leakage current in an off-state is about 1 nA. When a1-fF-storage capacitor is used, the retention period is about severalmicroseconds, and thus the extremely frequent refresh operation need beperformed to hold data.

As for the point (2), in the example in Patent Document 1, although fourseparate capacitors are used, data cannot be distinguished for eachcapacitor, and thus only four levels of data (or nine levels even whennegative potentials are used at the time of writing) can be stored,which is inefficient.

In view of the above, it is an object of one embodiment of the presentinvention to increase a storage capacity of a memory module per unitarea.

Further, it is another object of one embodiment of the present inventionto provide a memory module with small power consumption.

By adopting a multilevel and stacked-layer capacitor, the storagecapacity of a memory module per unit area is increased.

One embodiment of the present invention is a semiconductor memory devicecomprising: a bit line; a word line; n (n is a natural number of 2 ormore) data lines; a memory cell including a transistor having a channelformed in an oxide semiconductor film and n capacitors with differentcapacitances, wherein one of a source and a drain of the transistor isconnected to the bit line, and the other of the source and the drain ofthe transistor is connected to one electrode of each of the ncapacitors, a gate of the transistor is connected to the word line, andeach of the other electrodes of the n capacitors is connected to acorresponding of the n data lines.

The loss of electric charges from a capacitor is caused by off-statecurrent of a transistor. The off-state current means a current flowingbetween a source and a drain when the transistor is in an off-state.Since the off-state current flows, electric charges stored in acapacitor are lost with time. In order to avoid such a phenomenon, withuse of a transistor with a small off-state current, the retention periodof potentials in the capacitor can be extended.

The off-state current of a transistor is caused by recombination ofcarriers in a semiconductor film Therefore, as the band gap of asemiconductor film is larger and/or as the amount of an impurity servingas a recombination center of carriers is smaller, the off-state currenthardly flows. For example, for a transistor, one of an oxidesemiconductor film, a silicon carbide film, and a gallium nitride filmwhich are highly purified and have a band gap of 2.5 eV or higher may beused. Note that a transistor using a silicon carbide film or a galliumnitride film is often a depletion-type, and thus its threshold isdifficult to control. For that reason, in this specification,transistors using oxide semiconductor films are described, becauseenhancement-type transistors using oxide semiconductors have beenreported and known.

In particular, an oxide semiconductor film can be easily formed with asputtering apparatus or the like, and a transistor using such an oxidesemiconductor film exhibits a small amount of off-state current;therefore, such an oxide semiconductor film is a suitable material forcarrying out the present invention. For example, the off-state currentof a transistor using an oxide semiconductor film of In—Ga—Zn—O is1×10⁻¹⁸ A or lower, the off-state current of a transistor using ahighly-purified oxide semiconductor film of In—Ga—Zn—O is 1×10⁻²¹ A orlower, and when the concentration of an impurity is lowered, theoff-state current becomes extremely low, 1×10⁻²⁴ A or lower. This valueis actually 1/10¹⁴ to 1/10⁸ of the off-state current of a transistorhaving a channel in a semiconductor substrate of silicon, and aretention period of charges in a capacitor is 10⁸-fold to 10¹⁴-fold.

As described above, a transistor with a small amount of off-statecurrent is used, and thereby electric charges in a capacitor can be heldlong without an increase in the frequency of a refresh operation.

In addition, the reduced frequency of the refresh operation leads to lowpower consumption.

Further, since the loss of electric charges hardly occurs, comparison ina slight difference between capacitances can be performed and suchcapacitors can be downsized, memory cells are also downsized, andthereby a memory module can be downsized or highly integrated.

Furthermore, a memory cell includes n capacitors with differentcapacitances, different n data lines are connected to the n capacitorsrespectively, so that a variety of combinations of storage capacitancesin the entire memory cell can be obtained. By reading the potential inaccordance with the storage capacitance, a multilevel memory cell can berealized.

For example, the capacitance of a first capacitor is 1, the capacitanceof a second capacitor is 2, and the capacitance of a k (k is a naturalnumber of n or less)-th capacitor is 2^(k-1). When the number ofcapacitors in a memory cell is n, the number of the combination ofpotentials held in the memory cell is 2^(n). In other words, a2^(n)-level memory cell can be obtained.

In addition, in accordance with one embodiment of the present invention,capacitors can be stacked. When capacitors are stacked, a small-areamemory cell can be formed, and thus the storage capacity of a memorymodule per unit area can be more increased.

Alternatively, memory cells may be stacked. With memory cells stacked,the storage capacitance of a memory module per unit area can be muchmore increased.

A multilevel and stacked-layer memory cell is employed, and thereby thestorage capacity of a memory module per unit area can be increased.

Further, the frequency of a refresh operation in a memory cell isreduced, and thereby the power consumption of a memory module can bereduced.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are circuit diagrams illustrating examples ofsemiconductor memory devices;

FIG. 2 is a circuit diagram illustrating an example of modularizing asemiconductor memory device;

FIGS. 3A and 3B are a cross-sectional view and a top view of a memorycell in a semiconductor memory device, respectively;

FIG. 4A is a cross-sectional view and FIGS. 4B to 4D are top views of amemory cell in a semiconductor memory device;

FIG. 5A is a cross-sectional view and FIGS. 5B to 5D are top views of amemory cell in a semiconductor memory device;

FIG. 6A is a cross-sectional view and FIGS. 6B to 6D are top views of amemory cell in a semiconductor memory device;

FIG. 7A is a cross-sectional view and FIGS. 7B to 7D are top views of amemory cell in a semiconductor memory device; and

FIGS. 8A to 8C are diagrams for explaining a conventional semiconductormemory device.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the following description, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways. Therefore, the present inventionis not construed as being limited to description of the embodiments. Indescription of the structures of the present invention with reference tothe drawings, the same reference numerals are used in common for thesame portions in different drawings. Note that the same hatch pattern isapplied to similar parts, and the similar parts are not especiallydenoted by reference numerals in some cases.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps or the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify thepresent invention.

Before the description of the present invention, the terms used in thisspecification will be briefly explained. In this specification, one of asource and a drain of a transistor is called a drain while the other iscalled a source in this specification. That is, the source and the drainare not distinguished depending on the potential level. Therefore, inthis specification, a portion called a source can be alternativelyreferred to as a drain.

Note that voltage refers to a potential difference between a certainpotential and a reference potential (e.g., a ground potential) in manycases. Accordingly, voltage, potential and a potential difference can bereferred to as potential, voltage, and a voltage difference,respectively.

Further, in this specification, even when the phrase “A is connected toB” is used, there is a case in which no physical connection between Aand B is made in an actual circuit and a wiring is just extended betweenA and B. For example, in the case of a circuit including aninsulated-gate field-effect transistor (MISFET), one wiring functions asgates of a plurality of MISFETs in some cases. In that case, one wiringwhich branches into gates may be illustrated in a circuit diagram. Evenin such a case, the expression “a wiring is connected to a gate” may beused in this specification.

Note that in this specification, in referring to a specific row, aspecific column, or a specific position in a matrix, a reference sign isaccompanied by a sign denoting coordinates as follows, for example: “atransistor Tr_a_b”, or “a bit line BL_(—) b”. In the case where a row, acolumn, or a position is not specified, the case where elements arecollectively referred to, or the case where the position is obvious, thefollowing expressions may be used: “a transistor Tr” and “a bit lineBL”, or simply “a transistor” and “a bit line”.

Embodiment 1

In this embodiment, examples of a structure and an operation of a memorycell that is one of semiconductor memory devices will be described withreference to FIGS. 1A and 1B.

FIG. 1A is a circuit diagram illustrating a 2^(n)-level memory cellwhich includes a bit line BL; a word line WL; data lines DL_1 to DL_n; amemory cell CL having a transistor Tr and capacitors C_1 to C_n; and asense amplifier SAmp.

A gate of the transistor Tr is connected to the word line WL, one of asource and a drain of the transistor Tr is connected to the bit line BL,the other thereof is connected to one side of each of the capacitors C_1to C_n, and the other sides of the capacitors C_1 to C_n are connectedto the data lines DL_1 to DL_n. For example, the capacitor C_1, thecapacitor C_2, and the capacitor C_n are connected to the data lineDL_1, the data line DL_2, and the data line DL_n, respectively. The bitline BL is connected to the sense amplifier SAmp.

As the transistor Tr, a transistor with a small amount of off-statecurrent is used. Specifically, for example, a highly-purified oxidesemiconductor film having a wide band gap of 2.5 eV or higher may beused for an active layer of the transistor. Due to a wider band gap andfewer recombination centers of carriers, the amount of the off-statecurrent of the transistor can be small.

The transistor with a small amount of off-state current is used for amemory cell, and thus the loss of electric charges stored in a capacitorcan be inhibited. Therefore, the retention period of electric charges isextended, and the frequency of the refresh operation can be reduced,which can lead to reduction of power consumption. In addition, theinhabitation of the loss of electric charges makes it possible todecrease the capacitance of a capacitor and downsize of the memory cell,as compared with a conventional memory cell in a DRAM.

Further, the capacitance of each capacitor is adjusted, so that aplurality of amounts of electric charges held in the memory cell can beprovided. That is, a multilevel memory cell can be formed.

For example, the capacitance of the capacitor C_k (k is a natural numberof n or less) is 2^(k-1) fold of the capacitance of the capacitor C_1,so that the number of the combination of electric charges held can be2^(n). Note that in accordance with the number of n, the area of acapacitor should be enlarged or a capacitor insulating film should bethinned. For that reason, depending on the number of n, there is adisadvantage in downsizing. In addition, when the amount of electriccharges held is small, reading of potentials is difficult in some cases;therefore, n is preferably set in an appropriate range. For example, nis 2 to 8, preferably 3 to 5.

Note that the capacitance of the capacitor C_1 having the smallestcapacitance may range from 0.1 fF to 1 fF. With use of an oxidesemiconductor film for a transistor in a memory cell, even theabove-described capacitor with a small capacitance can hold electriccharges for a long period.

For example, with reference to FIG. 1B, a writing method underconditions where n is 3, the capacitance of the capacitor C_1 is 1 fF,the capacitance of the capacitor C_2 is 2 fF, and the capacitance of thecapacitor C_3 is 4 fF will be described.

Writing on the capacitor C_1 to C_3 can be conducted independently. Forexample, the bit line is set to a predetermined potential VDD (apotential ample to charge the capacitor), and VH is applied to the wordline. At this time, potentials of the data lines DL_1 to DL_3 may becontrolled. The capacitor connected to the data line having a potentialof VDD is not charged, while the capacitor connected to the data linehaving a capacitance of a ground potential, GND (reference potential),is charged to hold electric charges depending on the capacitance. Inthis specification, VH is a voltage having a potential higher than thevoltage that is obtained by adding VDD to a threshold voltage (Vth) ofthe transistor. Table 1 shows the combination of the voltages and thepotentials of the bit line (BL), the word line (WL), and the data lines(DL_1 to DL_3), and the storage capacitances (SC).

TABLE 1 BL WL DL_1 DL_2 DL_3 SC [fF] VDD VH GND GND GND 7 VDD VH VDD GNDGND 6 VDD VH GND VDD GND 5 VDD VH VDD VDD GND 4 VDD VH GND GND VDD 3 VDDVH VDD GND VDD 2 VDD VH GND VDD VDD 1 VDD VH VDD VDD VDD 0

As shown in Table 1, when n is 3, a 3-bit (8 levels) writing can beperformed. The case where n is 3 is only shown in the table; the samecan be applied to a case where n is more than 3. In other words, inaccordance with one embodiment of the present invention, writing can beperformed with the combination of 2^(n).

The potential of the memory cell CL can be held under the conditionswhere the potential of the bit line BL is a floating potential (float),the potential of the word line WL is GND or less than a thresholdvoltage, and the potentials of the data lines DL_1 to DL_3 are GND.

Next, a reading method will be described.

At the time of reading, the potentials of the data lines DL_1 to DL_3are set at GND and the potential of the bit line BL is set at anappropriate potential, e.g., VR. Next, the potential of the word line WLis set at VH, so that the potential of the bit line BL is shifted todata_CL in accordance with the held potential. Here, the data_CL can beexpressed in Equation 1.

$\begin{matrix}{{data\_ CL} = \frac{\begin{matrix}{{{VR} \cdot {C\_ BL}} + {{VDD} \cdot}} \\\left( {{{{f(1)} \cdot {C\_}}1} + {{{f(2)} \cdot {C\_}}2} + {{{f(3)} \cdot {C\_}}3}} \right)\end{matrix}}{{C\_ BL} + {{C\_}1} + {{C\_}2} + {{C\_}3}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In Equation 1, C_BL represents a capacitance of the bit line BL. Thef(1) to f(3) correspond to electric charges held in the capacitors C_1to C_3, respectively. When electric charges are held in the capacitorsC_1 to C_3, “1” is given to the f(1) to f(3), whereas when electriccharges are not held in the capacitors C_1 to C_3, “0” is given to thef(1) to f(3).

By detecting data_CL with use of the sense amplifier SAmp, 3 bits (8levels) of data can be read. The case where n is 3 is only describedhere; the same can be applied to a case where n is more than 3. In otherwords, in accordance with one embodiment of the present invention,reading can be performed with the combination of 2^(n). In that case,Equation 2 can be obtained as expansion of Equation 1.

$\begin{matrix}{{data\_ CL} = \frac{\begin{matrix}{{{VR} \cdot {C\_ BL}} + {{VDD} \cdot}} \\\left( {{{{f(1)} \cdot {C\_}}1} + {{{f(2)} \cdot {C\_}}2} + \ldots + {{f(n)} \cdot {C\_ n}}} \right)\end{matrix}}{{C\_ BL} + {{C\_}1} + {{C\_}2} + \ldots + {C\_ n}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In Equation 2, the f(n) corresponds to electric charges held in thecapacitor C_n. When electric charges are held in the capacitor, “1” isgiven to the f(n), whereas when electric charges are not held in thecapacitor, “0” is given to the f(n).

In a conventional transistor having a channel in a semiconductorsubstrate of silicon, a large amount of off-state current flows and thusthe potential cannot be held; therefore, a multilevel memory cell isdifficult to obtain. In contrast, when a transistor with a small amountof off-state current is used and a plurality of capacitors and capacitorwirings connected to the plurality of capacitors are provided, a2^(n)-level memory cell can be realized.

This embodiment can be combined with any of the other embodiments asappropriate

Embodiment 2

In this embodiment, a memory module having a rows and b columns usingthe memory cells described in Embodiment 1 will be described withreference to FIG. 2, as one example of semiconductor memory devices.

FIG. 2 illustrates a memory module including an address line ADL, a dataline DL, bit lines BL_1 to BL_(—) b, word lines WL_1 to WL_a, ANDcircuits, an output OUT, reading circuits 10_1 to 10 _(—) b, switchingcircuits 20_1 to 20 _(—) b, and memory cells CL_1_1 to CL_a_(—) b.

The address lines ADL are connected to the reading circuits 10_1 to 10_(—) b and the switch circuits 20_1 to 20_b via AND circuits. The datalines DL are each connected to a corresponding capacitor in the memorycells CL_1_1 to CL_a_b via the switch circuits 20_1 to 20_b. The bitline BL_1 is connected to one of a source and a drain of a transistor ineach of the memory cells CL_1_1 to CL_a_1 and to the output OUT via thereading circuit 10_1. Similarly, the bit line BL_2 and BL_b areconnected to one of a source and a drain of a transistor in each of thememory cells CL_1_2 to CL_a_2 and one of a source and a drain of atransistor in each of the memory cells CL_1_b to CL_a_b, respectivelyand are connected to the output OUT via the reading circuit 10_2 and thereading circuit 10_b respectively. The word line WL_1 is connected togates of the transistors in the memory cell CL_1_1 to CL_1_b. Similarly,the word line WL_2 and WL_a are connected to gates of the transistors inthe memory cells CL_2_1 to CL_2_b and gates of the transistors in thememory cells CL_a_1 to CL_a_b, respectively.

The memory cell CL_1_1 to CL_a_b may have the same structure as thememory cell CL described in Embodiment 1.

The reading circuits 10_1 to 10_b may be for example, sense amplifiers.

The switch circuits 20_1 to 20_b may be, for example, analogue switches.

Note that one or more registers may be provided between the switchcircuits 20_1 to 20_b and the memory cell CL_1_1 to CL_1_b. As describedin detail later, provision of the registers allows writing column bycolumn, which leads to increase of the writing speed.

Four data lines DL are used in this embodiment; however, the number ofthe data lines is not limited to four. The number may be selectedsuitably in accordance with the number of capacitors included in thememory cells CL_1_1 to CL_a_b.

Six address lines ADL are used in this embodiment; however, the numberof the address lines is not limited to six. The number may be selectedsuitably in accordance with the number of the memory cells constitutinga memory module.

Next, a writing method of data in the memory module in FIG. 2 isdescribed.

Data writing is performed on every memory cell. For example, when dataare written into the memory cell CL_a_b, the potential of the bit lineBL_b is set at the potential VDD, and VH is applied to the word lineWL_a. At this time, by applying appropriate high potentials (VH orhigher, for example) to the bit lines and the data lines which areconnected to memory cells not to be written, the transistors except oneincluded in the memory cell CL_a_b. stay in off-state. Then, the switchcircuit 20_b may be turned on with use of the address line ADL, bycontrol of the potential of the data line DL. In this manner,data_CL_a_b can be written into the memory cell CL_a_b. This operationis performed on every memory cell, and thus data (data_CL_1_1 todata_CL_a_b) can be written into all memory cells.

Alternatively, data writing is performed row by row. In this case, astructure in which a register is provided between a switch circuit and amemory cell as described above may be employed. As a specific datawriting method, for example, a method with which data are writtencollectively into the memory cells CL_a_1 to CL_a_b of the a-th rowsharing the word line WL_a is described. With the address lines ADL,only the switch circuit 20_1 is turned on, and the potentials of thecontrolled data lines DL are held in the register. Then, with theaddress line ADL, only the switch circuit 202 is turned on, and thepotentials of the controlled data lines DL are held in the register.After repetition of this step, with the address lines ADL, only theswitch circuit 20_b is turned on, and the potential of the controlleddata lines DL are held in the register. After that, in the state thatthe switch circuits 20_1 to 20_b are off, the potentials of the bitlines BL_1 to BL_b are set at the potential VDD, and VH is applied tothe word line WL_a, so that the memory cells CL_a_1 to CL_a_b can besupplied with data in accordance with the potentials held in theregister. This step is performed row by row, so that data can be writtenin the memory module including the memory cells on the a rows and the bcolumns.

Then, a reading method of data from the memory module in FIG. 2 isdescribed.

Data reading is performed on every memory cell. For example, when dataof the memory cell CL_a_b are read, the switch circuit 20_b is turned onwith the address line ADL and the potentials of the data lines DL areall set at GND and VR is applied to the bit line BL_b. Further, byapplying appropriate high potentials (VH or higher, for example) to thebit lines and the data lines which are connected to memory cells not tobe read, the transistors except one included in the memory cell CL_a_b.stay in off-state. Then, when the word line WL_a is set at VH, thepotential of the bit line BL_b is shifted to data_CL_a_b. The potentialis read in the reading circuit 10_b. This step is performed on everymemory cell, so that data of the memory module having memory cells onthe a rows and the b columns can be read.

The switch circuit and the reading circuit are connected to each other,and thus the potential to turn on the switch circuit may be a referencepotential of the reading circuit. With such a structure, the number ofwirings can be decreased, so that the memory module can be downsized orhighly integrated.

In accordance with one embodiment of the present invention, a memorymodule having a large capacity in which a plurality of 2^(n)-levelmemory cells are connected can be formed.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 3

In this embodiment, a memory cell that is one of semiconductor memorydevices will be described with reference to FIGS. 3A and 3B.

FIG. 3A is a cross-sectional view of a semiconductor memory deviceincluding a region 300 having a semiconductor substrate 331 and a firstinsulating film 332; a second insulating film 302 over the region 300;an island-shaped semiconductor film 306 provided over the secondinsulating film 302; a conductive film 308 and a conductive film 309which are partially in contact with the semiconductor film 306; a thirdinsulating film 312 provided over the semiconductor film 306, theconductive film 308, and the conductive film 309; a conductive film 304overlapped with the semiconductor film 306 with the third insulatingfilm 312 interposed therebetween; conductive films 341 to 344 formedwith the same layer as the conductive film 304; and a fourth insulatingfilm 316 provided to cover the third insulating film 312, the conductivefilm 304, and the conductive films 341 to 344.

As the semiconductor film 306, a highly-purified semiconductor filmhaving a band gap of 2.5 eV or higher is used. For example, an oxidesemiconductor film, a silicon carbide film, a gallium nitride film, orthe like may be used.

As a material used for the oxide semiconductor film, any of thefollowing materials may be used: a four-component metal oxide such as anIn—Sn—Ga—Zn—O-based material; three-component metal oxides such as anIn—Ga—Zn—O-based material, an In—Sn—Zn—O-based material, anIn—Al—Zn—O-based material, a Sn—Ga—Zn—O-based material, anAl—Ga—Zn—O-based material, and a Sn—Al—Zn—O-based material;two-component metal oxides such as an In—Zn—O-based material, aSn—Zn—O-based material, an Al—Zn—O-based material, a Zn—Mg—O-basedmaterial, a Sn—Mg—O-based material, and an In—Mg—O-based material, andan In—Ga—O-based material; an In—O-based material; a Sn—O-basedmaterial; a Zn—O-based material; and the like. In addition, any of theabove materials may contain silicon oxide. Here, for example, anIn—Ga—Zn—O-based material means an oxide containing indium (In), gallium(Ga), and zinc (Zn), and there is no particular limitation on thecomposition ratio. Further, the In—Ga—Zn—O-based material may contain anelement other than In, Ga, and Zn.

In particular, In—Sn—Zn—O-based materials are preferred for high speedoperation of a semiconductor memory device, because such materials canprovide high field effect mobility.

As the oxide semiconductor film, a thin film formed using a materialrepresented by the chemical formula, InMO₃(ZnO)_(m) (m>0), may be used.Here, M represents one or more metal elements selected from the group ofGa, Al, Mn, and Co. For example, M may be Ga, Ga and Al, Ga and Mn, Gaand Co, or the like.

The oxide semiconductor film can be formed by a sputtering method, a PLDmethod, a spray method, or the like.

In particular, when an oxide semiconductor film with high purity and fewdefects is formed by a sputtering method, the partial pressure of oxygenat the film formation is preferably 10% or more. In addition, thetemperature of the film formation is set in the range of 200° C. to 450°C., so that the concentration of impurities (such as hydrogen) in thefilm can be reduced.

Further, heat treatment is performed after the film formation, andthereby an oxide semiconductor film with higher purity and fewer defectscan be formed. Specifically, the heat treatment may be performed underthe conditions that the temperature is set at 150° C. or higher andlower than a strain point of the substrate, preferably from 250° C. to450° C., the atmosphere is highly purified nitrogen, oxygen or rare gas,or a mixed atmosphere of nitrogen, oxygen and/or a rare gas, and theprocess time is from 6 minutes to 24 hours. The process time may belonger than 24 hours; however, if the process time is too long, the costeffectiveness becomes low. Preferably, after the heat treatment isperformed under a nitrogen atmosphere, another heat treatment isperformed under an oxidative atmosphere (an atmosphere including oxygen,ozone, nitrous oxide, or the like at 10 ppm or more) without changingthe temperature. In this manner, purity can be heightened and oxygendeficiencies can be reduced.

Although not illustrated, the region 300 may include at least any one ofcircuits such as a sense amplifier circuit, a register circuit, and ananalogue switch circuit, and wirings. Here, the semiconductor substrate331 may be a substrate containing semiconductor such as a silicon wafer,a silicon carbide substrate, a gallium nitride substrate, a galliumarsenide substrate, a germanium substrate, or an SOI (Silicon OnInsulator) substrate. In addition, the first insulating film 332 isprovided to separate such circuits and wirings from the memory cell, orthe second insulating film 302 may also have the function of the firstinsulating film 332. The surface of the region 300 may be planarized bychemical mechanical polishing (CMP) or the like.

The transistor Tr includes the second insulating film 302 as a baseinsulating film; the semiconductor film 306 as an active layer; theconductive film 308 and the conductive film 309 as a source electrodeand a drain electrode; the conductive film 304 as a gate electrode; andthe third insulating film 312 as a gate insulating film. Note that thetransistor Tr may have any structure as appropriate without beinglimited to the structure illustrated in FIGS. 3A and 3B.

The second insulating film 302 may be a single layer or a stacked layerof a silicon oxide film, a silicon oxynitride film, a silicon nitrideoxide film, a silicon nitride film, an aluminum oxide film, and/or thelike. Since the second insulating film 302 serves as the base insulatingfilm of the transistor Tr, it is preferably an insulating film fromwhich oxygen is released by heating.

The phrase “oxygen is released by heating” means that the amount ofreleased oxygen which is converted into oxygen atoms is 1.0×10¹⁸atoms/cm³ or higher, preferably 3.0×10²⁰ atoms/cm³ or higher in thermaldesorption spectroscopy (TDS).

Here, a method in which the amount of released oxygen is measured bybeing converted into oxygen atoms using TDS analysis will now bedescribed.

The amount of released gas in TDS analysis is proportional to theintegral value of a spectrum. Therefore, the amount of released gas canbe calculated from the ratio between the integral value of a spectrum ofan insulating film and the reference value obtained from a standardsample. The reference value of a standard sample refers to the ratio ofthe density of atoms of a predetermined element contained in the sampleto the integral value of its spectrum.

For example, the number of the released oxygen molecules (N_(O2)) froman insulating film can be found based on Equation 3 with the TDSanalysis results of a silicon wafer containing hydrogen at apredetermined density which is the standard sample and the TDS analysisresults of the insulating film. Here, all spectra having a mass numberof 32 which are obtained by the TDS analysis are assumed to originatefrom an oxygen molecule. CH₃OH, which is given as a gas having a massnumber of 32, is not taken into consideration on the assumption that itis unlikely to be present. Further, an oxygen molecule including anoxygen atom having a mass number of 17 or 18 which is an isotope of anoxygen atom is also not taken into consideration because the proportionof such a molecule in the natural world is minimal.N_(O2)═N_(H2)/S_(H2)×S_(O2)×α  (Equation 3)

N_(H2) is the value obtained by conversion of the number of hydrogenmolecules desorbed from the standard sample into densities. S_(H2) isthe integral value of a spectrum when the standard sample is subjectedto TDS analysis. Here, the reference value of the standard sample is setto Nm/Sm. S_(O2) is the integral value of a spectrum when the insulatingfilm is subjected to TDS analysis. α is a coefficient influencing theintensity of the spectrum in the TDS analysis. For details of theEquation 3, Japanese Published Patent Application No. H6-275697 isreferred to. Note that the amount of released oxygen from the insulatingfilm is measured with a thermal desorption spectroscopy apparatusproduced by ESCO Ltd., EMD-WA1000S/W using a silicon wafer containinghydrogen atoms at 1×10¹⁶ atoms/cm³ as the standard sample.

Further, in the TDS analysis, oxygen is partly detected as an oxygenatom. The ratio between oxygen molecules and oxygen atoms can becalculated from the ionization rate of the oxygen molecules. Note that,since the above a includes the ionization rate of the oxygen molecules,the number of the released oxygen atoms can also be estimated throughthe evaluation of the amount of the released oxygen molecules.

Note that N_(O2) is the amount of the released oxygen molecules. For theinsulating film, the amount of released oxygen when converted intooxygen atoms is twice the amount of the released oxygen molecules.

In the above structure, the insulating film from which oxygen isreleased by heating may be oxygen-excess silicon oxide (SiO_(X) (X>2)).In the oxygen-excess silicon oxide (SiO_(X) (X>2)), the number of oxygenatoms per unit volume is more than twice the number of silicon atoms perunit volume. The number of silicon atoms and the number of oxygen atomsper unit volume are measured by Rutherford backscattering spectrometry.

By supplying oxygen from the base insulating film to the oxidesemiconductor region, the density of an interface state between the baseinsulating film and the oxide semiconductor region can be reduced. As aresult, electric charges or the like which may be produced due to anoperation of a transistor can be inhibited from being captured at theinterface between the base insulating film and the oxide semiconductorregion, and thereby a transistor with less degradation in electriccharacteristics can be provided.

Further, charges are caused due to oxygen deficiency in the oxygensemiconductor region in some cases. In general, a part of oxygendeficiencies in an oxide semiconductor region serves as a donor togenerate an electron that is a carrier. As a result, the thresholdvoltage of a transistor is shifted negatively. This tendency occursremarkably in oxygen deficiencies caused on the back channel side. Notethat the term “back channel” in this specification refers to thevicinity of an interface on the base insulating film side of the oxidesemiconductor region. Oxygen is sufficiently supplied from the baseinsulating film to the oxide semiconductor region, whereby oxygendeficiencies in the oxide semiconductor region which causes the negativeshift of the threshold voltage can be reduced.

In other words, when oxygen deficiencies are caused in the oxidesemiconductor region, it is difficult to inhibit trapping of a charge atan interface between the base insulating film and the oxidesemiconductor region. However, by providing an insulating layer fromwhich oxygen is released by heating as the base insulating film, theinterface state between the oxide semiconductor region and the baseinsulating film and the oxygen deficiencies in the oxide semiconductorregion can be reduced and the influence of the trapping of a charge atthe interface between the oxide semiconductor region and the baseinsulating film can be made small.

Examples of materials used for the conductive film 308 and theconductive film 309 include metals, alloys, or metal nitrides of thefollowing elements: aluminum, titanium, chromium, nickel, copper,yttrium, zirconium, molybdenum, silver, tantalum, and tungsten. Further,a transparent conductive material including indium oxide, tin oxide, orzinc oxide may be used. Further, a stacked-layer structure of suchmaterials may be employed.

In addition, the conductive film 309 serves as one electrode of each ofthe capacitors C_1 to C_4.

The third insulating film 312 may be, for example, formed with a stackedlayer or a single layer using silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, aluminum oxide, hafnium oxide,Yttria-stabilized zirconia, and/or the like. For example, the thirdinsulating film 312 may be formed by a thermal oxidation method, a CVDmethod, a sputtering method, or the like. As the third insulating film312, an insulating film from which oxygen is released by heating may beused. With use of such an insulating film from which oxygen is releasedby heating for the third insulating film 312, defects generated in thesemiconductor film 306 can be repaired and electric characteristics ofthe transistor can be inhibited from being degraded.

In addition, the third insulating film 312 serves as a capacitorinsulating film of each of the capacitors C_1 to C_4.

The materials used for the conductive film 304 and the conductive films341 to 344 may be similar to those of the conductive film 308 and theconductive film 309.

The conductive films 341 to 344 serve as the other electrode of each ofthe capacitors C_1 to C_4. In other words, the capacitances of thecapacitors C_1 to C_4 are determined depending on the material and thethickness of the third insulating film 312, and the areas of theconductive films 341 to 344.

The fourth insulating film 316 may be, for example, formed with astacked-layer or a single layer using silicon oxide, silicon oxynitride,silicon nitride oxide, silicon nitride, or the like. For example, thefourth insulating film 316 may be formed by a thermal oxidation method,a CVD method, a sputtering method, or the like. Further, an organicmaterial such as polyimide or acrylic may be used.

FIG. 3B is a top view of the memory cell illustrated in FIG. 3A. Notethat the other components than the conductive film 308, the conductivefilm 309, the conductive film 304, and the conductive films 341 to 344are omitted for simplification.

The conductive film 308 serves as the bit line BL, the conductive film304 serves as the word line WL, and the conductive films 341 to 344serve as the data line DL_1 to DL_4.

Since, in accordance with this embodiment, the off-state current of thetransistor Tr is small, the storage capacitances of the capacitors canbe reduced. In addition, the change of the potentials held in the memorycell is minute, and thus a slight difference between the potentials canbe read, and thus a multilevel memory cell can be formed.

Note that, in this embodiment, the memory cell includes four capacitorsand four data lines; however the numbers of the capacitors and datalines are each not limited to four, and can be selected as appropriatedepending on the storage capacity needed.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 4

In this embodiment, a memory cell having an area reduced due to astacked structure, which is different from the memory cell in Embodiment3, will be described.

FIG. 4A is a cross-sectional view of the memory cell. In thisembodiment, the memory cell includes the region 300, a region 471 abovethe region 300, a region 472 above the region 471, and a region 473above the region 472.

The region 471 includes the transistor Tr and the capacitor C_1, theregion 472 includes the capacitor C_2, and the region 473 includes thecapacitor C_3. Note that the capacitors C_1, C_2, and C_3 includeconductive films 441, 442, and 443, insulating films 491, 492, and 493,and conductive films 481, 482, and 483, respectively as theircomponents. The insulating films 491 to 493 may be formed from differentmaterials and with different thicknesses so that the capacitances of thecapacitors C_1 to C_3 can be controlled. The conductive films 481 to 483are connected to each other through contact holes.

The surfaces of the region 300 and the regions 471 to 473 may beplanarized by CMP or the like. The planarization of the surface of eachregion can decrease adverse effect of steps in each region.

FIG. 4B, FIG. 4C, and FIG. 4D are top views of the region 473, theregion 472, and the region 471, respectively, which are seen from above.

In this embodiment, the regions having the capacitors are stacked toform a three-layer structure; however, the regions having the capacitorsmay be stacked to form a structure with four layers or more layers.

With such a structure, the area of the memory cell can be reduced.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 5

In this embodiment, a memory cell having an area which is more reduceddue to a stacked structure of capacitors and provision of a plurality ofcapacitors in one layer, which is different from the memory cells inEmbodiments 3 and 4, will be described.

FIG. 5A is a cross-sectional view of the memory cell. In thisembodiment, the memory cell includes the region 300, a region 571 abovethe region 300, a region 572 above the region 571, and a region 573above the region 572.

The region 571 includes the transistor Tr and the capacitors C_1 andC_2, the region 572 includes the capacitors C_3 and C_4, and the region573 includes the capacitor C_5. Note that the capacitors C_1 and C_2include conductive films 541 and 542, an insulating film 591, and aconductive film 581 as their components; the capacitors C_3 and C_4include conductive films 543 and 544, an insulating film 592, and aconductive film 582 as their components; the capacitor C_5 includes aconductive film 545, an insulating film 593, and a conductive film 583as its components. The insulating films 591 to 593 may be formed fromdifferent materials and with different thicknesses so that thecapacitances of the capacitors C_1 to CS can be controlled. Theconductive films 581 to 583 are connected to each other through contactholes.

The surfaces of the region 300 and the regions 571 to 573 may beplanarized by CMP or the like.

FIG. 5B, FIG. 5C, and FIG. 5D are top views of the region 573, theregion 572, and the region 571, respectively, which are seen from above.

In this embodiment, the regions having the capacitors are stacked toform a three-layer structure; however, the regions having the capacitorsmay be stacked to form a structure with four layers or more layers,without being limited to the three-layer structure.

The structure in this embodiment where one region includes a pluralityof capacitors makes it possible to provide a memory cell storing morelevels of data in the same area, as compared with the structure inEmbodiment 4. In other words, the area of the memory cell per unit areacan be more reduced.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 6

In this embodiment, a memory cell having a much more reduced area, whichis a different one from the semiconductor memory devices in Embodiments3 to 5, will be described. In the memory cell, a region 600 includes astep portion and a capacitor is embedded in the step portion, so thatthe area of the memory cell is reduced. The structure of this embodimentis a so-called trench structure and a capacitor with a small area and alarge capacity can be formed.

FIG. 6A is a cross-sectional view of the memory cell. In thisembodiment, the memory cell includes the region 600, a region 671 abovethe region 600, and a region 672 above the region 671.

The region 600 has a structure similar to that of the region 300, exceptthat the region 600 includes the step portion and the capacitor. Thecapacitor CS includes an insulating film 690, a conductive film 680, anda conductive film 645 as its components.

The region 671 includes the transistor Tr and the capacitors C_1 andC_2, the region 672 includes the capacitors C_3 and C_4, and the region600 includes the capacitor C_5. Note that the capacitors C_1 and C_2include conductive films 641 and 642, an insulating film 691, and aconductive film 681 as their components; the capacitors C_3 and C_4include conductive films 643 and 644, an insulating film 692, and aconductive film 682 as their components; and the capacitor CS includes aconductive film 645, an insulating film 690, and a conductive film 680as its components. The insulating films 690 to 692 may be formed fromdifferent materials and with different thicknesses so that thecapacitances of the capacitors C_1 to C_5 can be controlled. Theconductive films 680 to 682 are connected to each other through contactholes.

The surfaces of the region 600 and the regions 671 and 672 may beplanarized by CMP or the like.

FIG. 6B, FIG. 6C, and FIG. 6D are top views of the region 672, theregion 671, and the region 600, respectively, which are seen from above.

In this embodiment, the regions having the capacitors are stacked toform a three-layer structure; however, the regions having the capacitorsmay be stacked to form a structure with four layers or more layerswithout being limited to the three-layer structure.

Since the region 600 includes the step portion and the capacitor isprovided in the step portion, the area occupied with the capacitor inthe memory cell can be reduced, whereby the area of the memory cell canbe much more reduced than those in Embodiments 4 and 5.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 7

In this embodiment, a semiconductor memory device having a small areawill be described, which is another embodiment of a semiconductor memorydevice having the trench structure as described in Embodiment 6. In thesemiconductor memory device, a region 700 includes a step portion andthe capacitor is formed over the step portion as illustrated in FIG. 7A.

FIG. 7A is a cross-sectional view of the memory cell. In thisembodiment, the memory cell includes the region 700, a region 771 abovethe region 700, and a region 772 above the region 771.

The region 700 has a structure similar to that of the region 300, exceptthat the region 700 includes the step portion 711 and the capacitor. Thecapacitor C_5 includes an insulating film 790, a conductive film 780 anda conductive film 745 as its components.

The region 771 includes the transistor Tr and the capacitors C_1 andC_2, the region 772 includes the capacitors C_3 and C_4, and the region700 includes the capacitor C_5. Note that the capacitors C_1 and C_2include conductive films 741 and 742, an insulating film 791, and aconductive film 781 as their components; the capacitors C_3 and C_4include conductive films 743 and 744, an insulating film 792, and aconductive film 782 as their components; and the capacitor CS includes aconductive film 745, an insulating film 790, and a conductive film 780as its components. The insulating films 790 to 792 may be formed fromdifferent materials and with different thicknesses so that thecapacitances of the capacitors C_1 to C_5 can be controlled. Theconductive films 780 to 782 are connected to each other through contactholes.

The surfaces of the region 700 and the regions 771 and 772 may beplanarized by CMP or the like.

FIG. 7B, FIG. 7C, and FIG. 7D are top views of the region 772, theregion 771, and the region 700, respectively, which are seen from above.

In this embodiment, the regions having the capacitors are stacked toform a three-layer structure; however, the regions having the capacitorsmay be stacked to form a structure with four layers or more layerswithout being limited to the three-layer structure.

Since the region 700 includes the step portion and the capacitor isprovided in the step portion, the area occupied with the capacitor inthe memory cell can be reduced, whereby the area of the memory cell canbe much more reduced than those in Embodiments 4 and 5.

This embodiment can be combined with any of the other embodiments asappropriate.

This application is based on Japanese Patent Application serial No.2010-261470 filed with the Japan Patent Office on Nov. 24, 2010, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a substrate; acapacitor over the substrate; and a transistor over the capacitor,wherein the transistor includes an oxide semiconductor film, a pair ofelectrodes electrically connected to the oxide semiconductor film, agate insulating film, and a gate electrode, wherein the capacitor isformed by a capacitor electrode, a capacitor wiring and an insulatingfilm interposed between the capacitor electrode and the capacitorwiring, wherein it is designed that a potential of the capacitorelectrode is always substantially the same as that of one of the pair ofelectrodes, and wherein it is configured that, when data is written, apotential of the capacitor wiring is changed in accordance with thedata.
 2. The semiconductor device according to claim 1, wherein thesubstrate is a silicon wafer.
 3. The semiconductor device according toclaim 1, wherein the gate electrode is formed over the oxidesemiconductor film.
 4. The semiconductor device according to claim 1,wherein the insulating film is formed over the capacitor electrode, andthe capacitor wiring is formed over the insulating film.
 5. Thesemiconductor device according to claim 4, wherein the capacitor wiringis a same material as the gate electrode.
 6. The semiconductor deviceaccording to claim 4, wherein it is configured that, when the data iswritten, a potential of the other of the pair of electrodes is changedin accordance with the data.
 7. The semiconductor device according toclaim 4, wherein a width of the capacitor wiring is wider than a widthof the capacitor electrode.
 8. The semiconductor device according toclaim 4, wherein the capacitor electrode is formed over a depressedportion of the substrate.
 9. The semiconductor device according to claim4, further comprising another capacitor formed by the one of the pair ofelectrodes, the gate insulating film, and another capacitor wiring. 10.A semiconductor device comprising: a substrate; a capacitor over thesubstrate; and a transistor over the capacitor, wherein the transistorincludes an oxide semiconductor film, a pair of electrodes electricallyconnected to the oxide semiconductor film, a gate insulating film, and agate electrode, wherein the oxide semiconductor film overlaps thecapacitor, wherein the capacitor is formed by a capacitor electrode, acapacitor wiring and an insulating film interposed between the capacitorelectrode and the capacitor wiring, wherein it is designed that apotential of the capacitor electrode is always substantially the same asthat of one of the pair of electrodes, and wherein it is configuredthat, when data is written, a potential of the capacitor wiring ischanged in accordance with the data.
 11. The semiconductor deviceaccording to claim 10, wherein the substrate is a silicon wafer.
 12. Thesemiconductor device according to claim 10, wherein the gate electrodeis formed over the oxide semiconductor film.
 13. The semiconductordevice according to claim 10, wherein the insulating film Is formed overthe capacitor electrode, and the capacitor wiring is formed over theinsulating film.
 14. The semiconductor device according to claim 13,wherein the capacitor wiring is a same material as the gate electrode.15. The semiconductor device according to claim 13, wherein it isconfigured that, when the data is written, a potential of the other ofthe pair of electrodes is changed in accordance with the data.
 16. Thesemiconductor device according to claim 13, wherein a width of thecapacitor wiring is wider than a width of the capacitor electrode. 17.The semiconductor device according to claim 13, wherein the capacitorelectrode is formed over a depressed portion of the substrate.
 18. Thesemiconductor device according to claim 13, further comprising anothercapacitor formed by one of the pair of electrodes, the gate insulatingfilm, and another capacitor wiring.